1. Field of the Invention
The present invention relates to a rotary machine of the type including a plurality of impellers which are fixedly mounted on a main shaft and more particularly to an improvement of or relating to a rotor or impeller of the rotary machine such as a steam turbine, gas turbine or the like which is shrink fit onto the main shaft.
2. Description of the Prior Art
During rotation of impellers in a centrifugal type compressor under high load at a high rotational speed, thrust force generated by a high centrifugal force as well as a pressure differential appearing across each of the impellers is exerted on the impellers. There is a necessity for correctly maintaining the center of rotation during operation of the rotary machine without any occurrence of slippage of the impellers on the main shaft under the effect of thrust force irrespective of how much the inner diameter of the impellers expand in the radial direction under the effect of centrifugal force. To prevent an occurrence of slippage of the impellers on the main shaft there has been exclusively employed a method of fixedly mounting impellers on the main shaft by shrink fitting instead of the conventional methods using spline, key, press fitting or the like means.
To facilitate understanding of the invention one of the conventional methods for fixedly mounting impellers on the main shaft is shown in FIG. 2. The inner peripheral portion C of each of the impellers 1 is shrink fit onto the main shaft 2 and the portions D of a sleeve 3 separated from the impeller 1 are also shrink fit onto the latter.
As is well known, the latest rotary machine tends to operate under conditions of higher rotational speed and higher load and therefore there is greater concern in failing to operate the rotary machine in the stable state in which impellers are rigidly mounted on the main shaft in accordance with the conventional shrink fitting method. As the impellers 1 are rotated at a higher rotational speed, a higher centrifugal force is generated, resulting in increased expansion of the inner peripheral portion C in the radial direction. To ensure that torque is properly transmitted from the main shaft 2 to the impeller 1 with the main shaft 2 being rotated against forces exerted on the main shaft 2 due to pressure exerted on the impellers, it is required that a sufficiently high frictional force exist between the outer peripheral surface of the main shaft 2 and the inner peripheral surface of each of the impellers 1. To meet the requirement there is a necessity to provide a sufficient dimension of shrink fit. As the dimension of shrink fit increases, stress in the tangential direction in the inner peripheral wall of the impeller during shrink fitting increases, resulting in an increased differential of stress between operation of the compressor and during stoppage of operation of the same. This causes the inner peripheral portion of each of the impellers to expand and contract every time the compressor starts its operation and stops and distribution of surface pressure active on the impellers in the axial direction varies. This leads to displacement of the impellers 1 relative to the main shaft 2 causing an occurrence of vibration of the rotor.
For instance, when it is assumed that the outer diameter of the impeller 1 is 400 mm, the inner diameter of the same is 130 mm and the number of revolutions is 15,000 r.p.m., the inner diameter expands by a dimension of about 2/1000.times.(inner diameter) when they are rotated. If a distance of shrink fit is predetermined to be 2.5/1000.times.(inner diameter), only an effective distance of shrink fit of 0.5/1000.times.(inner diameter) exists during rotation of the impellers. The above-noted effective distance of shrink fit is not sufficient for assuring reliable transmission of torque from the main shaft 2 to the impellers 1 and producing a frictional force which is sufficient to satisfactorily stands differential pressure across each of the impellers. Further, to achieve shrink fitting of a dimension of 2.5/1000.times.(inner diameter) it is required that each of the impellers is heated up to an elevated temperature in the range of 300.degree. to 350.degree. C. in a furnace. Any requirement for an increased dimension of shrink fit is limited by heating temperature.